Awesome, thanks for your help. That’s what I was thinking about the MaxMind warning, but I only had time to read briefly before I logged off yesterday.
The CSV file is generated from Salesforce and it has one email address on each line, no commas or spaces. When I paste it into a basic text editor, it looks totally fine; no extra characters or anything. I also tried with a new CSV with a single email address and had the same error.